What alerts are they ? Is it an email alert that you are talking about or alerts on your alert view within Solarwinds?
1. Check Solarwinds app event logs in Event Viewer to see if any errors were logged on alerting service during that period.
2. Check your Solarwinds poller performance during those hours.
If its an email alert that you are talking about then the issue could be with your SMTP Server which is sending out the mail, if it isn't an email alert ignore this post and check for other details mentioned on my previous post.